For converting into Comparative degree, the sentence structure should be altered to make a direct comparison between the two places: Direct Sentence: Nainital is not so hot as Agra. Comparative Degree: Nainital is less hot than Agra.
Explanation: In comparative degree, we often replace "not so + adjective + as" with "less + adjective + than" to show the difference in degree between two objects.
\[
\boxed{\text{Nainital is less hot than Agra.}}
\]